tmhd82,
I hear you about the door panels! I don't think mine still fit quite right, but I don't want to mess with them again. 2016 Colorado 4X4, Z71.
I didn't have the tools or time to do the connector, pins, etc. route. I just used 20ga wire, some in-line crimp connectors, blade tips for the mirror, zip ties and patience. Might take you an afternoon once you see how one side goes in.
It's possible to route the wires to the mirror connectors without removing the back plastic cover on the mirror. I found that out the hard way. Just take your time. I ran my wires to the mirror through the "rubber doughnut plug" that the motor control wires go through. I did put some of my findings on the heated mirror forum subject.